Infantile spinal muscular atrophy. Unusual fiber typing and distribution in a muscle biopsy.

A right gastrocnemius muscle biopsy of a 13-month-old floppy male infant, which appeared at a more advanced age to suffer from an infantile spinal muscular atrophy, showed unusual histochemical changes: the chequer-board distribution was replaced by three groups of muscle fibers with a same mean narrow diameter of 12.5 micrometer. The smallest groups could easily be recognized as type I and type IIB fibers, while the largest group, involving more than 75% of the whole biopsy, revealed an intermediate hybrid fiber population, which would be classified as type I, if based on their phosphorylase and myofibrillar ATPase activities alone. The pathogenesis of this unusual finding is discussed.